The article states that the PS5 is responsible for significantly more gaming hours, so a large number of PS4 users are likely mainly using their device for streaming or similar. Hell, I still have my PS2 and 3 in my closet since they’re my only dvd and blu ray players.
Typically when looking at upgrades, I upgrade the cpu, mobo, and ram together. Since a new mobo normally means support for faster cpu/ram (and possibly a new socket, so a new cpu is needed anyways), it makes sense to upgrade the 3 at the same time.